The sliding surface of a sliding member is typically coated by a hard carbon coating to increase the wear resistance. An amorphous carbon known as diamond-like carbon (DLC) is used as the carbon coating. Structurally, DLC is in essence a combination of diamond bonding (SP3 bonding) and graphite bonding (SP2 bonding) as the carbon bonding. Consequently, DLC has diamond-like hardness, wear resistance, thermal conductivity, and chemical stability while also having graphite-like solid lubricity. DLC is therefore suitable for use as a protective coating of a sliding member, such as an automotive part.
Since a hard carbon coating has high internal stress and extremely high hardness, however, the high carbon coating does not adhere closely to the base member when formed thickly and tends to peel. To address this problem, known techniques such as those in patent literature (PTL) 1 to 3 form a layered carbon coating on a sliding surface by alternately layering a high-hardness carbon coating and a low-hardness carbon coating. With these techniques, the stress of the layered carbon coating as a whole is alleviated by the low-hardness carbon coating, thereby achieving close adhesion of the layered carbon coating to the base member.
PTL 1 discloses a DLC multi-layer coating in which a low-density carbon coating with a density of 2.2 g/cm3 or less and a high-density carbon coating with a density of 2.3 g/cm3 to 3.2 g/cm3 are layered alternately, the thickness T1 of the low-density carbon coating is 0.4 nm to 30 nm, the thickness T2 of the high-density carbon layer is 0.4 nm to 10 nm, and T1/T2 is 0.2 to 5.
PTL 2 discloses a piston ring in which the sliding surface is coated by a carbon-based coating. The coating is a layered coating with two or more layers, the layers being of two types with a different hardness. The hardness difference between the two types of layers is 500 HV to 1700 HV, the high-hardness layer has a thickness equal to or greater than the thickness of the low-hardness layer, and the thickness of the coating as a whole is 5.0 μm or more.
PTL 3 discloses a diamond-like carbon coating formed on a substrate with a bonding layer therebetween. A highly tough diamond-like carbon coating layer is formed to have four or more layers by alternately layering, on the upper layer of the bonding layer, a soft carbon coating that has a hardness of 500 Hv to 2000 Hv and is substantially free of hydrogen and a hard carbon coating that has a hardness of 2000 Hv to 4000 Hv and is substantially free of hydrogen. A lubricating diamond-like carbon coating layer with a hardness of 500 Hv to 2000 Hv and including hydrogen is then formed on the top layer, i.e. on the upper layer of the highly tough diamond-like carbon coating layer.